The technology of lithium ion batteries is advancing fast. Meanwhile, lithium ion batteries have been widely applied in the fields of power source and power storage. Comparing with the batteries used in miniature portable electronic devices (such as mobile phones), the batteries used in the fields of power source and power storage are requested to provide features such as high output power and prolonged service life. Thus, the batteries for power source or power storage normally need to be connected in parallel or in series to form a battery pack, which is of high value and expensive. Therefore, batteries applied in fields of power source and power storage demand higher reliability and higher consistency. Since failure of any individual battery of a battery pack would induce the entire battery pack to malfunction or even cause a safety problem, the batteries to be used in battery packs should be tested and sifted to guarantee operational consistency of each battery. The factors to evaluate the consistency of batteries include capacity, voltage, discharge voltage platform, internal resistance, and self-discharge rate, wherein self-discharge rate is particularly important but hard to evaluate. In a system where batteries cannot be charged and discharged uniformly, the operational consistency of the batteries will have great changes after long time storage or long time usage because of different self-discharge rates. In an industrial standard test method for self-discharge, a fully-charged battery (SOC (State Of Charge) 100%) is left unused for 1 month, and then the charge retention rate thereof is measured. A test method for self-discharge commonly used by the manufacturers includes the following steps: charging a battery to a fully-charged state (SOC 90-100%) or a half-charged state (SOC 40-60%), storing the battery at a room temperature or a high temperature, and evaluating the self-discharge level of the battery via detecting the voltage difference between the pre-storage battery and post-storage battery. However, the conventional test methods for self-discharge have a disadvantage that a voltage-capacity relationship of the lithium ion battery is unclear. For example, there is no obvious voltage-capacity relationship in the fully-charged state and the half-charged state of a lithium iron phosphate battery. Therefore, the abovementioned methods cannot achieve the desired test effect unless the batteries are stored for a long time. Thus, the abovementioned methods are unsuitable for mass-production. Further, the higher the charged level of batteries, the greater the safety risk for storing or assembling the batteries. Furthermore, the self-discharge test of fully-charged or half-charged batteries spends a lot of time, normally 15-30 days. Thus, each cycle of batch production requires a long time, which causes high carrying costs, raises the total cost and customers are not satisfied with delivery date. If the test period is shortened, the batteries of high self-discharge are hard to be picked out. Then, the customers will complain about the unqualified products. Traditionally, the reliability of battery packs conflicts with carrying cost and fast delivery. A China Patent No. CN101764259A disclosed a self-discharge test method using a condition of SOC 50-90% and storage for 7-14 days, which uses higher SOC batteries and longer storage time. While picking those batteries out, there are both qualified batteries and unqualified batteries for self-discharge. With high SOC, a high self-discharge battery is easily to heat up and has risks of burning spontaneously, which may generate a temperature of over 130° C. High temperature may melt the internal separation membranes of neighboring batteries, which may further cause a chain reaction of internal short circuit of the batteries. Many fires occurred in battery manufacturers are attributed to spontaneous heating and burning of batteries. On the other hand, while the batteries are stored with SOC 50-90%, the voltage thereof is within the discharge platform section and relatively stable. In such a case, the voltage of the battery is hard to reveal the self-discharge level of the battery, and the batteries with high self-discharge cannot be all identified even though longer test time is used. Thus, there are always some unqualified batteries escaping from testing and mixed in battery packs. Dangers of malfunction, fire and explosion are hidden in the abovementioned unqualified batteries.